Does this flat 5 sided face count as a square?
It is a pentagon or the more common general term in 3d, a n-gon. You probably want to avoid these for anything realtime such as games or animations since they do not deform or smooth well, make adding more detail cumbersome and games are more optimal with triangulated meshes. 

As a simple flat object, it's fine if you are done with the model but again as a rule of thumb, try to stick with quads and tris.

To resolve, you can either:


Use the join tool (select both vertices and press J).
Use the knife tool (K, select a star point and drag to cut, use Enter to confirm) to create a tri from the extra vertex or create quads by continuing the edge flow downwards.